Hybrid vehicles have a combustion engine and a battery powered electric motor that are selectively engaged to drive the vehicle. The combustion engine has maximum rated torque output and a delayed response to a demand for increased torque. Fuel consumption can be reduced by providing the vehicle with a smaller engine, however, the maximum rated torque of the smaller engine results in slower acceleration for the vehicle if the smaller engine is used alone.
Improved responsiveness in hybrid vehicles that have smaller engines can be provided by adding a torque boost from the electric motor while the engine is driving the vehicle. Adding torque from the electric motor discharges the battery and reduces the range of vehicle operation in the battery powered driven mode.
The transmission for the hybrid vehicle may include a hydraulic control system that utilizes a separate pump motor that adds weight to the vehicle and discharges the battery. The electric motor may be used to provide line pressure to the transmission but a minimum level of line pressure must be maintained to protect the transmission.
The above problems and others are addressed by this disclosure as summarized below.